Field of the Invention
The present invention relates to an antireflection coating formed by a multi-layer film.
Description of the Related Art
Optical apparatuses such as surveillance cameras include ones required to be capable of not only visible light image capturing, but also near-infrared light image capturing in darkness such as night. Such image capturing in darkness is made possible using airglow radiated due to a chemical reaction and an electromagnetic reaction in the atmosphere. The airglow, which is generated in a wavelength range from a visible range to a near-infrared range, has a peak in a wavelength range from 1450 nm to 1800 nm in the near-infrared range (see “Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays” (Committee on Developments in Detector Technologies; National Research Council, 2010) pp. 25).
On the other hand, a surface of a light-transmissive member such as a lens used in an image capturing optical system is provided with, in order to reduce a reflectance thereat, an antireflection coating formed by a multi-layer film in which multiple dielectric thin films formed by evaporation (deposition) are layered. Furthermore, an antireflection coating including an uppermost layer whose material has a lower refractive index than a refractive index of 1.38 of magnesium fluoride used for the evaporated film can provide a higher antireflection performance. As low refractive index materials, inorganic materials such as silica and magnesium fluoride and organic materials such as silicon resin and non-crystalline fluorine resin are known. These materials can reduce their refractive indices by forming voids in their layers.
In the optical apparatuses capable of the visible light image capturing and the near-infrared light image capturing, a reduction of the reflectance of the light-transmissive member such as the lens not only reduces a generation of unwanted light such as flare and ghost, but also increases an amount of transmitted light to obtain more image information.
Japanese Patent Laid-Open No. 2013-250295 discloses an antireflection coating having an antireflection effect from the visible range to the near-infrared range. This antireflection coating has a twelve-layer structure in which high refractive index films and low refractive index films are alternately layered, and its twelfth layer as an uppermost layer is formed of an extremely low refractive index material whose refractive index is in a range from 1.20 to 1.29.
However, the antireflection coating disclosed in Japanese Patent Laid-Open No. 2013-250295 has a reflectance characteristic that its reflectance increases in a wavelength range of 1600 nm or more and that provides a reflectance of 1% or more in a wavelength range near 1700 nm. Since the airglow has its peak in the wavelength from 1450 nm to 1800 nm as described above, a level of an antireflection performance approximately the same as that of the antireflection coating disclosed in Japanese Patent Laid-Open No. 2013-250295 is insufficient for the airglow.